The present invention relates to apparatus and methods for supplying auxiliary steam in a combined cycle system employing a gas turbine having steam cooling and particularly relates to a combined high pressure/low pressure system for extracting auxiliary steam from the combined cycle system and supplying the auxiliary steam to an end user, for example, steam seals and sparging devices for starting one or more additional combined cycle systems.
In a typical combined cycle system, a gas turbine combusts a fuel/air mixture which expands to turn the turbine and drive a generator for the production of electricity. The hot gases of combustion exhaust into a heat recovery steam generator in which water is converted to steam in the manner of a boiler. Steam thus produced drives a steam turbine, typically comprising high, intermediate and low pressure turbines, in which additional work is extracted to drive a further load such as a second generator for producing additional electric power. In some configurations, the gas and steam turbines drive a common generator and, in others, drive different generators.
In a conventional combined cycle system where additional similar combined cycle systems are employed, auxiliary steam is often generated by a host system and applied to such second or additional systems for use during startup, for example, to provide auxiliary steam to the steam seals and sparging devices of such additional units. Auxiliary steam is typically extracted from the exhaust of the high pressure steam turbine and supplied directly to a header which, in turn, supplies auxiliary steam to the sub-systems of the additional unit(s). Auxiliary steam may also be utilized for other end purposes, for example, as process steam in co-generation applications.
Combined cycle systems also typically employ air-cooled gas turbines. The auxiliary steam conventionally extracted from the high pressure steam turbine exhaust is sufficiently cool for direct auxiliary use without attemperation. In more recent advanced design combined cycle systems, however, steam-cooled gas turbines are employed. Thus, steam from the high pressure steam turbine flows through the steam-cooling circuit of the gas turbine and the spent cooling steam is reheated for return to the steam turbine, for example, the intermediate pressure steam turbine. Low pressure steam is, of course, also supplied from the low pressure heater in the heat recovery steam generator to the steam turbine. Given the necessity of maintaining the flow of cooling steam through the gas turbine, it no longer remains practical to use steam from the high pressure steam turbine for use as auxiliary steam because to do so reduces the necessary cooling steam for the gas turbine, causing overheating and attendant problems. Consequently, the problem addressed is the provision of auxiliary steam in a combined cycle system employing a steam-cooled gas turbine.
In accordance with an embodiment of the present invention, there is provided a combined cycle system employing a steam-cooled gas turbine wherein low pressure steam from the low pressure superheater of the heat recovery steam generator and normally supplied to the intermediate pressure steam turbine is diverted, at least in part, for flow through a first conduit by opening a pressure-responsive control valve. The diverted flow of steam flows into a header for flowing steam to another combined cycle system during startup to steam seals, for use as sparging steam, and, when applicable, steam turbine cooling steam at startup. When additional steam flow is required, spent cooling steam exhausting from the cooling circuit of the gas turbine may also be diverted in part from its return to the reheater by opening a pressure responsive high pressure control valve for flowing steam in a second conduit. These first and second steam flows are combined in a third conduit, also containing a flow nozzle for measuring the flow. A water attemperator is disposed in the third conduit and a thermocouple measures the steam temperature in the header. The attemperator is responsive to the measured flow and header steam temperature to reduce the temperature of the combined flows when the flow in the third conduit obtains a predetermined minimum flow and the header temperature exceeds a A predetermined temperature. Thus, low pressure steam from the heat recovery steam generator and high pressure spent cooling steam are combined and attemperated at a predetermined flow and header steam temperature to produce an auxiliary steam flow. The attemperator is consequently used only when a predetermined minimum steam flow and temperature have been established. The low pressure flow does not usually require attemperation and establishes the predetermined minimum flow. The valve positions and, consequently, the flow through the first and second conduits is determined by the auxiliary steam pressure in the header to which the auxiliary steam is supplied.
There are many different types of end uses for the generated auxiliary steam. For example, auxiliary steam can be used as process steam for co-generation purposes. Another use of the auxiliary steam is for starting other off-line combined cycle units. That is, in power generating stations having multiple combined cycle units, it is desirable to supply auxiliary steam from one operating unit to another unit during startup of the second unit. Auxiliary steam is necessary during startup of a second combined cycle unit, for example, for purposes of activating the steam seals, sparging the condenser and commencing delivery of cooling steam to the steam turbine when the gas turbine and steam turbine use a common generator. Otherwise, another source of steam must be utilized, for example, from an additional boiler, which may not always be available. Consequently, the auxiliary steam generated by the host unit can be supplied to a second unit header for supplying auxiliary steam to the steam seals and sparging devices and for cooling the steam turbine of the second unit, if applicable. Because the auxiliary steam temperature may be too high for use in certain devices, attemperation or temperature reduction is desirable.
It will be appreciated from the foregoing-described system that one or more additional off-line units can be started using auxiliary steam from the host unit as described and wherein the additional units may be started seriatim with short time intervals between starts. Additionally, the high pressure/low pressure system of this invention does not impact the steam-cooling requirements of the gas turbine of any one or more of the combined cycle units. This is significant because it is critically important that the steam flow for the cooling circuit in each gas turbine be maintained for proper cooling of the steam-cooled components thereof. Additional advantages of the present system include the use of low temperature steam from the LP HRSG system to establish a minimum flow level before the attemperator is used for the higher temperature supplemental steam. This minimizes the potential for water-related damage due to inadequate mixing of the steam and attemperating water. Further advantages include the use of reduced cost headers, for example, carbon steel headers instead of alloyed headers necessary for higher temperature steam. Still further, each unit is provided its own steam turbine cooling, when a common generator is used, and sealing steam during full speed no-load operation after a load rejection.
In a preferred embodiment according to the present invention, there is provided apparatus for supplying auxiliary steam to an end user comprising a combined cycle system including a steam-cooled gas turbine, a steam turbine and a heat recovery steam generator in heat transfer relation with hot products of combustion from the gas turbine, the heat recovery steam generator including a low pressure superheater for generating steam and driving the steam turbine, a reheater for flowing spent cooling steam from the gas turbine to the steam turbine, a low pressure control valve for diverting at least a portion of the low pressure steam generated by the low pressure heater for flow through a first conduit for auxiliary steam use, a high pressure control valve for diverting at least a portion of the spent high pressure cooling steam from the gas turbine for flow thereof through a second conduit, a third conduit for receiving the low pressure and high pressure steam flows of the first and second conduits, respectively, and defining an auxiliary steam conduit for flow thereof to an end user and an attemperator in the third conduit for controlling the temperature of the auxiliary steam.
In a further preferred embodiment according to the present invention, there is provided in a combined cycle system having a steam cooled gas turbine, a steam turbine and heat recovery steam generator in heat transfer relation with hot products of combustion from the gas turbine and including a heater for generating steam and driving the steam turbine, a method of generating auxiliary steam for use by an end user, comprising the steps of diverting at least a portion of the steam generated by the heater supplied to the steam turbine for flow through a first conduit, diverting at least a portion of the spent cooling steam from the gas turbine for flow through a second conduit, combining the steam flows through the first and second conduits in a third conduit for flowing auxiliary steam to an end user and reducing the temperature of the steam in the third conduit.